Repulpable, water repellant paperboard

ABSTRACT

Paperboard having a water-repellant coating which includes a polymer matrix/wax/pigment mixture and is applied to the paperboard as an aqueous formulation. The polymer matrix includes polymer chains which are ionically cross-linked through pendant carboxylic acid groups to provide, together with the wax and pigment components, a substantially continuous, water-repellant film which more readily breaks down under the alkaline conditions of repulping. The coated board also exhibits low slip and resistance to blocking, and is particularly well-suited in corrugated box constructions used in shipping containers for perishable food items such as meats, poultry, fishery products, fruits and vegetables. The pigmented or unpigmented coating formulations are also used as an overcoat over preprinted paperboard surfaces to simultaneously provide scuff and water resistance.

SPECIFICATION

This application is a continuation-in-part of application Ser. No.08/128,582 filed on Sep. 28, 1993, now an allowed case, which isincorporated herein by reference.

1. Field of Invention

The present invention relates to water repellant paper products such ascoated paperboard and the like. More particularly, the invention relatesto a water-repellant board having a pigmented polymeric/wax coatingwhich exhibits improved repulpability, release characteristics,resistance to blocking and printability. The transparent coatings haveapplication as an overcoat on preprinted paperboard surfaces to alsoprovide scuff resistance.

2. Background Art

Perishables, such as ice-packed chicken, meat, fishery products, fruitsand vegetables are typically shipped in boxes or containers formed ofcorrugated paperboard which has been coated with a water repellantmaterial. Conventional water-repellant coatings include natural andsynthetic waxes and various synthetic organic compounds such as ethylenevinyl acetate. Such coatings improve the water resistance and decreasethe water vapor transmission rate of the board so that the box is bettersuited for shipping perishables in a damp environment than an uncoatedboard. However, once used, such coated shipping boxes are difficult torecycle, since the coating tends to hold the fibers together and makesthem difficult to separate. Thus, conventional wax-based coatings cannotbe separated to the degree necessary to enable satisfactory repulping ofthe board and typically contain particles of the coating ("stickies")which stick to the papermaking equipment, wire, presses and dryer cansand make the resulting furnish unsuitable for papermaking.

Since it has generally not been economical to repulp wax-coated board,used shipping boxes and the like made from water-repellant linerboardhave typically been dumped into a landfill or incinerated rather thanrecycled. Not only is this practice wasteful, it is also environmentallyundesirable and a contributing factor to the growing solid wastedisposal problem.

In addition, conventional water repellant linerboard can be difficult towork with, especially under warm conditions, since conventionalwater-repellant coatings may become tacky causing stacked or contiguoussheets of coated board to block or stick together. This blocking oftenpersists after the sheets are cooled and detrimentally affects handlingof coated linerboard blanks and boxes made of coated linerboard.

Thus it would be desirable to find an alternative for conventional waxcoated and known polymeric water-repellant coatings that have comparablewater resistance properties but also have the added benefit ofrepulpability.

U.S. Pat. No. 4,010,307 to Canard et al. discloses a gloss coatingcomposition comprising an aqueous dispersion containing a binder and apigment. The pigment comprises 70-95% calcium carbonate and 5-30% of anon-film forming synthetic polymer having particle sizes in the range of0.05 to 0.3 microns.

U.S. Pat. No. 4,265,977 to Kawamura et al. discloses a paper coatingcomposition comprising an aqueous dispersion of styrene-butadienecopolymer binder in combination with conventional pigment materials anddispersants. The coating formulations contain pigments including clay,talc, calcium carbonate, aluminum hydroxide, barium sulfate as well aspigment dispersing agents and antifoaming agents.

U.S. Pat. No. 4,301,210 to Yasuda et al. discloses an aqueous papercoating composition containing pigment and binder, where the binder maycomprise styrene-butadiene copolymer and the pigment material maycomprise clay, aluminum hydroxide, calcium carbonate, etc.

U.S. Pat. No. 4,980,024 to Payne et al. relates to a low slip linerboardused in corrugated boxes having increased surface friction. Thelinerboard is coated with a composition consisting essentially of anacrylamide, glycerine and an antiskid aqueous silica sol.

U.S. Pat. No. 3,985,937 to Fife discloses polymer latex emulsions as alaminating adhesive in the production of corrugated board. Thecompositions include a styrene-butadiene latex emulsion, clay, paraffinwax and a wetting agent. The wax component is preferably a wax emulsionand is present in the range of 0.1 to 4% based on the amount of polymerlatex solid in the composition.

U.S. Pat. No. 4,117,199 to Gotoh discloses a repulpable paper having acoating of synthetic rubber latex and a wax. The coating is producedfrom an aqueous emulsion containing 100 parts of butadiene-methylmethacrylate copolymer latex and 5-100 parts of a wax emulsion (paraffinand polyolefin waxes). The emulsion coating is dried at an elevatedtemperature the same as or higher than the melting point of the wax.

The art does not provide aqueous pigmented/polymer/wax formulationscoated on linerboard substrates to produce a paper product with lowslip, water resistent and heat resistent characteristics that is easilyrecycled. The invention is directed to the provision of such recyclablepaper stocks. It would be appreciated that advantage over knowncompositions would be obtained by providing aqueous coatings includingpigments, a carboxylated polymeric binder, a wax emulsion and an ioniccrosslinking agent to provide a coated paper product with low slip andeasy recyclability.

Accordingly it is an object of the present invention to provide awater-repellant paperboard especially well-suited for use in theconstruction of corrugated boxes for shipment of perishable foods.

A further object of the present invention is to provide awater-repellant board of the character described which exhibits improvedrepulpability as compared with conventional wax coated boards.

An additional object of the present invention is to provide awater-repellant paperboard of the character described which exhibits lowslip and improved resistance to blocking.

Another object of the present invention is to provide a water-repellantpaperboard of the character described which is economical to produce andto recycle.

Still another object of the present invention is to provide a coatingformulation useful as an overprint varnish over preprinted substratesurfaces to simultaneously provide scuff and water resistance.

Having regard to the foregoing and other objects and advantages, thepresent invention is directed to a water repellant paperboard whichexhibits improved properties, particularly repulpability. The paperboardsheet is especially well suited for use as linerboard in the manufactureof corrugated boxes for shipping and storing food, particularly frozenfoods such as ice-packed chicken, meat, fishery products, fruits andvegetables. The invention is useful in demanding applications whichrequire a combination of high water repellency, high wet strength anddurability under wet conditions for use in food containers.

DISCLOSURE OF INVENTION

In general, the invention is directed to a fibrous paper web such aspaperboard having a water-repellant coating or film which includespigments, wax and a polymer matrix of polymer chains ionicallycross-linked through pendant carboxylic acid groups. The coating isapplied as an aqueous formulation which is dried to provide thewater-repellant coating on the paper web.

Under alkaline conditions, such as when contacted with repulping liquorscontaining NaOH or ammonia solutions, the ionic bonds in the coatingdisassociate to permit the film to be broken up in a more highly dividedform for enhanced repulpability as compared with conventional (wax)coatings. The wax particles confer water-repellency and, in combinationwith the pigments, enhance repulpability by interrupting the continuousfilm formed by the polymer matrix and limit the cohesive strength of thefilm.

Other objects, features, and advantages of the present invention willbecome apparent from the following detailed description of the best modeof practicing the invention when considered with reference to thedrawings, as follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-sectional view of a corrugated boardincorporating a repulpable, water-repellant paperboard according to apreferred embodiment of the present invention;

FIG. 2 is a perspective view of a corrugated container incorporating theboard of FIG. 1;

FIG. 3 is a diagrammatic view illustrating a process for recycling boardaccording to the present invention; and

FIG. 4 is a schematic model of the coating composition "barriermembrane".

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Co-pending application Ser. No. 08/128,582 filed on Sep. 28, 1993, whichis incorporated herein by reference, discloses aqueous coatingcompositions including carboxylated polymers, wax and crosslinkingagents. The present invention compositions are modifications andimprovements of these formulations.

As used herein, the term "water-repellant" merely refers to thehydrophobic character of the coating and its tendency to repel, block ornot significantly absorb or transmit water in normal use. Thus, the term"water-repellant" is intended to include "water-resistant" and otherterminology which connotes substantial as opposed to total or completewater blocking properties, and refers to a water-blocking property whichis sufficient for the intended use requiring a degree ofwater-repellency.

With reference now to the drawings in which like reference charactersdesignate like or similar parts throughout the several views, FIG. 1shows a cross-section of a portion of a corrugated board 10 which, inthe illustrated embodiment, is a combined construction of thedouble-face type having an inner linerboard 12 and an outer linerboard14 supported in substantially parallel, spaced-apart relation by acorrugating medium or fluting 16. The board 10 of the illustratedembodiment is particularly well-suited for the production of shippingcontainers, as in the production of the open-top container 20 of FIG. 2for shipment of frozen or meat, fruit, vegetables as well as ice-packedpoultry and the like. The board 10 may thus be visualized as a portionof a blank for making such a container, or a portion of the assembledcontainer.

The linerboards 12 and 14 each include interiorly facing surfaces 22 and24, respectively, which are adhesively secured to tips 25 of the fluting16 as by the application of a suitable adhesive to the tips of theflutes, such as an FDA approved starch based adhesive.

Each exteriorly facing surface 26 and 28 of linerboards 12 and 14includes a coating or film layer 30 and 32, respectively, whichcomprises pigments and wax mixed with a polymer matrix of polymer chainsionically cross-linked through pendant carboxylate groups, whichtogether with the wax confers water-repellency to the film but allowsthe coating to be repulpable under alkaline conditions. The coating isrelatively stable under neutral and acidic conditions, but has beenobserved to disassociate under alkaline conditions encountered duringrepulping so that particles of the coating which are liberated from thefibrous portion of the linerboard in repulping are more finely dividedin the resulting slurry.

The layers 30, 32 may be provided by applying through use of anysuitable coating process, such as a rod and/or air knife coater, anaqueous coating including a carboxylated polymer, pigments and a waxemulsion with an ionic cross-linking agent to the surfaces 26 and 28 oflinerboards 12 and 14.

The linerboard 14, corresponding to the outer surface of a container asillustrated in FIG. 2, may be preprinted on prior to the application ofcoating film layer 32. This overcoat layer, which alternatively may notcontain a pigment component, further provides scuff resistance to thecoated surface. A preferred way of applying the barrier coating on apreprinted substrate is to apply the coating in an overprint varnishstation on the printing press apparatus. The overprint varnish stationmay be a rod or blade coater with subsequent drying capabilities.

The polymer component of the formulations generally comprise from about25 to 45 percent by weight of the coating application (% by weight --drybasis). A preferred polymer for use in the coating is astyrene-butadiene (SB) copolymer polymerized with monomers havingcarboxylic acid pendant groups, e.g., acrylic acid and methacrylic acid.An especially preferred SB polymer for use in the invention is thecarboxylated styrene-butadiene latex available under the trade name RAP314NA from Dow Chemical Company of Midland, Mich. Another similar SBpolymer which may be used is available under the tradename 654NA, alsofrom Dow Chemical Company. The physical characteristics and propertiesof these commercially available materials are further described intechnical data sheets which are incorporated herein by reference. Theselatexes are believed to have a chemical formula as follows: ##STR1##

Other ionically cross-linkable polymers which may be used include, byway of example and not by way of limitation, polymers selected from thegroup consisting of vinylidene chloride/vinyl chloride, styrene-acrylic,styrene-butadiene, acrylic polymers, acrylic copolymers, polyvinylacetate, polyvinyl alcohol, ethylene-vinyl acetate, polyethylene vinylchloride, polyvinylidene chloride and starch. Illustrative of some ofthe chemical formulations of these polymers are listed below: ##STR2##

The polymers utilized in the coating compositions are carboxylated bycopolymerizing with a monomer having carboxylic acid pendant groups. Themonomers are generally an acrylate based monomer selected from the groupconsisting of an acrylic acid, methacrylic acid, itaconic acid, maleic(cis) acid, fumaric (trans) acid and other acrylate based monomers.Carboxylic acid pendent groups are necessary for the crosslinkingreaction to occur. The polymer molecules utilized in the compositionsare typically commercially available as carboxylated polymers.

The polymer chains are ionically cross-linked through pendent carboxylicacid groups with a crosslinking agent. The essentially ionic characterof the carboxylate bridge between the polymer chains is believed toconfer a high degree of stability to acids and water (essentiallyneutral) to form a superior water repellant and substantially continuousfilm on the paperboard which is not readily attacked by conditions ofnormal use. The crosslink is also believed to increase the effectiveglass transition temperature of the coating, so that contiguous sheetsof the coated paperboard are less likely to block.

The crosslinking agent component of the coating is selected from thegroup consisting of zinc oxide ammonium complexes and zirconium oxideammonium complexes. Preferably a metal oxide such as the zinc oxideammonium complex available under the trade designation Zinplex 15 fromUltra Additives, Inc. of Paterson, N.J. is used. Other suitablecrosslinking agents include ammonium zirconium carbonate crosslinkerssuch as the ammonium zirconium carbonate composition available under thetrade name HTI Insolubilizer 5800 M from Hopton Technologies, Inc.Albany, Oreg. The crosslinking component may comprise 2 to 30 parts byweight of the coating and preferably makes up about 2-10 percent of thecoating by weight (dry basis).

In an alternate embodiment, under certain conditions, the crosslinkingagent can be excluded from the coating formulation. As long as thecorrugated boards are produced and converted within two hours thecrosslinking agent is not necessary. Other similar coatings that alsomay be used under these conditions are disclosed in copending U.S.patent application Ser. No. 08/290,626 to Ma et al. entitled "REPULPABLEAQUEOUS BARRIER COATINGS AND RELATED PAPER STOCKS" filed concurrently,and are incorporated herein by reference.

The wax component of the coating is preferably provided by a lowmolecular weight paraffin-polyethylene emulsion such as a mixture of apolyethylene (molecular weight in the range of from about 500 to about2000), paraffin wax and an emulsifying agent. The polyethylene maycomprise from about 1 to about 10 weight percent of the wax emulsion andthe paraffin wax may comprise from about 30 to about 25 weight percent(wet basis). The emulsifying agent may comprise up to 7 weight percent,with the balance water (50-70%).

The wax emulsion may comprise from about 10 to about 30 weight percentof the coating (% by weight--dry basis). The % solids by weight of thecoating and wax emulsion are as follows:

    ______________________________________                                        MATERIAL        % SOLIDS   % WATER                                            ______________________________________                                        COATING         35-60      40-65                                              WAX EMULSION    30-50      50-70                                              ______________________________________                                    

A particularly preferred wax emulsion is the paraffin/polyethyleneemulsion available under the trade name Mobilcer 136 from Mobil OilCorporation of Fairfax, Va. Other suitable wax emulsions includeparaffin/microcrystalline wax emulsions such as the wax emulsion soldunder the trade designation Mobilcer J of Mobil Oil Corporation ofFairfax, Va. and the wax emulsion available under the trade nameMobilcer MTD 216 from the Mobil Oil Company or a paraffin wax emulsion,Mobilcer XMTD241, also available from Mobil Oil Company. Nopcote DS101,a synthetic wax emulsion, available from Henkel Corporation, Charlotte,N.C., may also be used. Nopcote DS101 is composed of 30-50% syntheticwax, up to 7% of emulsifying agents and 50-65% water (% by weight--wetbasis). In addition, wax emulsions based on carnauba wax,carnauba/paraffin wax, carnauba/microcrystalline andcarnauba/polyethylene resin may be used. The physical characteristicsand properties of these commercially available materials are furtherdescribed in technical data sheets which are incorporated herein byreference.

The pigments incorporated into the formulations are generally present inthe amount of 25 to 60% (% by weight--dry basis). The composition of thepigment dispersion is typically as follows:

    ______________________________________                                        % by weight (as-is) Component                                                 ______________________________________                                        40-65               Water                                                     up to 1             Dispersant                                                35-60               Pigment particles                                         ______________________________________                                    

The crosslinking agent, in the amount of 2-10%, is preferably added tothe pigment dispersion to prevent gelling. The % solids of the pigmentdispersion is generally 35-60%. Types of pigment dispersants includepolyacrylates, complex phosphates or mixtures of both. The particle sizeof the pigments are of the size that 85-90% pass through a 325 mesh,which is a wire mesh consisting of 325 wires per inch.

The following description describes the general procedure for producingthe pigment dispersions of the invention. Water, approximately 250 lbs,is charged to a Kady mill (volume 55 gallons) by Kinetic DispersionCorporation, Scarborough, Me., which is equipped with high shear andintensive grinding capability. Agitation is initiated and the dispersantwhich is approximately 0.6 lbs of tetrasodiumpyrophosphate (TSPP) and0.6 lbs of polyacrylate is added to the Kady Mill. The crosslinkingagent, 25 lbs of Zinplex 15 which is a zinc ammonium carbonate complex(15% solids), is then added. The mixture is stirred for approximately 3minutes. Finally, the pigment particles, such as MicaWhite 200-250 lbs,are added and stirred for 15 minutes. The pigment dispersion is thenready for blending with other components to form the final coatingformulation, including a defoamer, i.e. Colloid 682 from Rhone-Poulenc,Marietta, Ga., the polymeric binder, i.e. Dow 314, the wax emulsion,i.e. Nopcote DS-101, and a cellulosic gum thickener, i.e. Admiral 3089FSfrom Aqualon, Wilmington, Delaware. Although the procedure has beendescribed using specific materials the compositions are not limited tosuch and other suitable materials may be substituted.

The pigments are selected from the group consisting of aluminumtrihydrate, barium sulfate, calcium carbonate, mica (potassium aluminumsilicates), nepheline syenite (sodium potassium aluminosilicate), finelyground silica sand and other natural and synthetic type of silicates,talc (magnesium silicates), wollastonite (calcium metasilicates),bentonite (montmorillonite, smectite) and clay.

The pigments are generally available commercially under varioustradenames and from various manufacturers. Representative pigments thatmay be used include, but are not limited to, MicaWhite 200, availablefrom Franklin Minerals, Denver, Colo.; Mica C-4000, available from KMGMinerals, Kings Mountain, N.C.; Vantalc 6H and PDX 181 slurry, bothtalcs which are available from R. T. Vanderbilt, Norwalk, Connecticut;Black Hills Bond, bentonite, available from Black Hills Bentonite,Mills, Wyo.; and Opazil AS, bentonite, available from Albion Kaolin Co.,Hephziban, Ga. The physical characteristics and properties of thesecommercially available materials are further described in technical datasheets which are incorporated herein by reference.

Preferred pigments incorporated into the formulations include mica,talc, clay and bentonite and are considered "platelet" type of pigmentsbased on their particle shape, however, other types of pigments may alsobe used. When platelet type pigments are incorporated into thecomposition formulations improved water vapor barrier properties of thecoatings is observed. This is believed to be due to the presence of a"tortuous path" created by the pigments. FIG. 4 is a schematic modelillustrating the "barrier membrane" of the composition coating showingalignment of the pigment components.

As shown in FIG. 4, the coating 90 forms a substantially pin-hole freecontinuous film on the substrate surface 92. The film is comprised of aninterlocking network of polymer, wax, and pigments. The polymer,preferably styrene-butadiene, wax and pigment components are randomlyorganized and evenly distributed within the coating film formed on thesubstrate surface. As illustrated, the platelet pigments 98 are mostlyaligned parallel to the substrate surface due to the stress applied tothe coating during the application process. The rod or blade coatingessentially shears the coating and doctors the excess material off thesurface. Under high shear between the rod or blade and the papersubstrate, the platelet pigments are mostly aligned parallel to thesubstrate surface.

The good water vapor barrier properties of the invention coatings arebelieved to be provided by the presence of this "tortuous path". Thepermeability of barrier membranes and the theory of "tortuous path" arediscussed in an article entitled "Barrier Membranes" by E. L. Cussler etal., published in the Journal of Membrane Sciences, Vol. 38, 1988, pp.161-174, which is incorporated herein by reference. It is believed thatthe configuration of the polymer, wax and pigment components of thecoating extend the diffusion path length and lengthens diffusion timewhich thus improves water barrier properties.

Hydrophobicity of the pigment mixtures can be adjusted by using highlyhydrophobic pigments, i.e. talc, mica; medium hydrophobic pigments, i.e.wollastonite, nepheline syenite, finely ground silica and hydrophilicpigments, i.e. clay, bentonite, calcium carbonate, aluminum trihydrate,barium sulfate.

Typically, the pigments used in the invention coatings have relativelylow refractive index, meaning that the pigmented coatings will generallybe transparent. This has advantage when applied as an overcoat on apreprinted substrate or linerboard surface. Specifically, the mica, talcand bentonite pigments will not block the printing. If desired,whiteness of the coating can be adjusted by using various amounts oftitanium dioxide and calcium carbonate in combination with the otherpigments. In alternate overprint varnish applications the pigmentcomponent may be excluded.

The composition of the coatings 30 and 32 may further include thickenersto modify the viscosity of the composition for application of thecoating. A suitable thickener is the cellulose gum theological propertymodifier available under the trade designation Admiral 3089FS availablefrom Aqualon Company of Wilmington, Del. Sodium or ammoniumpolyacrylates may also be used as thickeners for the composition.

In addition, organic silicone free or silicone-based defoamers may beused. Preferred defoamers are an organic, silicone-free defoameravailable under the trade name Colloid 682 from Rhone-Poulenc ofMarietta, Ga. and an organic, silicone-free defoamer available fromWitco Chemical of New York, N.Y. under the trademark BubbleBreaker 748.These defoamers are preferably used in an amount ranging from about 0.05to about 0.5 percent by weight based on the total weight of the coating.Color pigments may also be added, such as to impart a whitish color foraesthetics.

Substrates in the present invention comprise a fibrous web of cellulosicmaterials and include a variety of coated and uncoated paper andpaperboard, including bleached or unbleached hardwood or softwood,virgin or recycled and clay-coated or uncoated forms of paper orpaperboard. Preferred application of the aqueous coatings are to thesurfaces of wet-strength or non-wet strength linerboards for use inagricultural box applications.

The coating is preferably applied at the rate of from about 2 to about 3pounds dry per 1,000 sq. ft. of board at a viscosity in the range of 400to 700 cps such that the coating will comprise from about 2 to about 3percent of the total weight of the board. Conventional board coated toprovide approximately the same water resistance typically has a waxcoating weight of between about 5-8 lbs/msf. Thus, a significantadvantage of the present invention is that the desired water resistancemay be realized with a coating provided in an amount which is only about25-35% of that required when conventional wax coatings are used.

The degree of penetration of the coatings into the substrate surface isdependent on the viscosity of the composition as well as the type ofsubstrate used. Generally the coatings compositions have a viscosity inthe range of 150-2500 cps and a solids content greater than 35%. The lowviscosities of the compositions results in little penetration into thesubstrate surface but enough for adhesion or binding of the coating tothe substrate surface. Optionally, a primer such as polyvinylidienechloride may be applied prior to the barrier coat to seal a poroussubstrate surface. Other primers used in the invention include awater-based dispersion of a polymer selected from the group comprisingacrylic polymers, acrylic copolymers, polyvinyl acetate, polyvinylalcohol, ethylene-vinyl acetate, polyethylene vinyl chloride,styrenebutadiene copolymers, polyvinylidene chloride or starch. Theprimer coat may further include a wax component, preferably comprising5-30 wt. % of the primer coat. The primer coat may further includepigments or mineral fillers, such as, but not limited to, aqueousdispersions of clay, calcium carbonate or mica. Typically, substratesused in the invention, which are preferably linerboards having a weightof 26-90 lbs/msf, do not require a primer coat. In all embodiments thebarrier coating adheres or binds to the substrate surface to form acontinuous film.

The cross-linking reaction occurs in the process of drying the coatedboard which may be carried out using forced hot air and conventionalcan-dryers as by threading the web with the coating thereon through astack of rotating cans, which advance the web through the dryersalternatively exposing opposite faces of the web to the hot surfaces ofthe cans. Preferably, the coated board is pre-dried before contactingthe surfaces of the dryer cans by non-contact heating such as forced hotair (temp. 200°-400° F.) for 5-15+ seconds to dry the coating to atleast a substantially non-sticky state prior to contact drying at thecan dryers.

The ionic bonding is believed to provide the coating with a polymermatrix represented generally by the formula P-S-CO₂ -M-O₂ C-S-P, whereinP represents the polymer as in the preferred styrene-butadiene polymer,S represents a polymer side chain, CO₂ and O₂ C represent carboxylicgroups and M represents a metal, such as zinc, from the cross-linkingagent (such as zinc oxide ammonium complex) providing the ioniccross-link or bridge between adjacent polymer chains.

The ionic crosslinking reaction for a styrene-butadiene acrylatecopolymer emulsion/zinc oxide ammonium complex is believed to be asfollows: ##STR3##

With respect to the above example, it is believed that substantially allof the free ammonia resulting from the cross-linking reaction isliberated as ammonia gas during drying, and is therefore not present inthe dried coating. In addition, it is believed that substantially all ofthe carbon dioxide and water reaction products are liberated duringdrying so that the dried coating is essentially void of reaction sideproducts.

The fibrous web of the linerboards 12 and 14 may be of conventionalconstruction including 1, 2, 3 or more plies (multi-ply), forming acomposite web. The board may further have a conventional thickness C offrom about 0.008 to about 0.03 inches and a weight of from about 26 toabout 90 pounds per 1,000 sq. ft.

In the case in which the linerboard is to be used in the manufacture ofcorrugated containers for shipping ice-packed perishables, which is ause for which the invention is particularly well-suited, the fibrous webof the linerboards 12 and 14 may desirably comprise a high wet-strengthpaperboard produced by any of the known techniques. Generally speaking,wet-strength paper is a paper which retains 15 percent or more of itsdry strength when thoroughly wetted with water.

A number of available paperboard products have wet strength in the rangeof 30 to 40 percent of the dry strength. Wet strength may be conferredby addition of wet-strength resins such as derivatives ofurea-formaldehyde, melamine-formaldehyde and polyamide-epichlorohydrinadded to the furnish stock after the last refiner in the papermakingmachine. The benefits of the present invention are important infacilitating the repulping of wet strength paper which has generallybeen considered more difficult to repulp due to the presence ofwet-strength resins. That is, although wet-strength paper is a materialof choice for corrugated containers for shipping perishables due to thepresence of water, the modification of the linerboard which confers wetstrength coupled with the presence of resin or wax surface-coatings hascompounded the difficulties in repulping this material. The surfacecoating of the present invention provides improved repulpability of suchcomposites and, thus, lessens the repulping problems inherent in the useof wet strength wax-coated board.

For packaging applications such as meat, fruit, and vegetables, non-wetstrength linerboard substrates such as 57 lb PL (pineliner) or 69 K(kraft) were coated with aqueous based barrier coating formulations. Forshipping meat, the inside of the corrugated box is preferably coated forobtaining properties such as water/grease resistance and releaseproperties at freezer conditions at 15°-30° F. In the cases of boxes forfruit and vegetables, both the inside and outside surfaces of thecorrugated boxes are preferably coated to provide water resistancesufficient to withstand the conditions encountered in the field,processing plants, storage, and distribution in cooler environment at35°-45° F.

Other alternate packaging embodiments include combined use of thepresent pigmented/wax/polymer coatings and use of the wax/polymercoatings described in copending Ser. No. 08/128,582. These latercoatings provide high slip and improved release characteristics to thecoated substrate surfaces. Thus, for instance, for shipping meat it maybe desirable to apply the wax/polymer coating on the inside of thecorrugated box for obtaining improved release properties at freezerconditions and coating the outside of the boxes with thepigmented/wax/polymer coating to provide low slip characteristicsdesirable during shipping. Other such variations and combinations arecontemplated by the invention and depend on the application and theproperties and characteristics desired for the packaging.

In all embodiments, for meat, fruit, and vegetables applications, theboard was observed to be readily repulpable with uncoated corrugatedcontainers at 100°-120° F. and pH of 7-8, which are the standardcommercial repulping conditions for recycled medium plant such asWaldorf Corporation in Minneapolis, Minn.

With further reference to FIG. 2, the container 20 may be erected from asingle blank (not shown) produced from the corrugated board 10 byfolding along score lines and/or gluing the blank in a manner well knownin the art. The coating layers 30 and 32 are compatible with the use oflap-gluing adhesives conventionally used for coated boards, and thecoating readily accepts water-based flexo inks.

The container 20 of the illustrated embodiment is of a type suitable forshipping iced perishables and includes a bottom panel 33 having anupwardly facing inner surface 34 and a downwardly facing outer surface36. A pair of side panels 38 and 40 extend upwardly from opposite sidesof the bottom panel 33 and include inwardly facing inner surfaces 42 and44, respectively, and outwardly facing outer surfaces 46 and 48,respectively. A pair of end panels 50 and 52 extend upwardly fromopposite sides of the bottom panel 33 adjacent the side panels 38 and40. The end panels 50 and 52 likewise include inwardly facing innersurfaces 54 and 56, respectively, and outwardly facing outer surfaces 58and 60.

Each inner surface 34, 42, 44, 54 and 56 of the container 20 may beprovided by surface 26 of the corrugated board 10 coated with layer 30.Similarly, each outer surface 36, 46, 48, 58 and 60 is provided bysurface 28 of the board 10 coated with coating layer 32. A top (notshown) may also be constructed of the board 10 of FIG. 1 to includeshort depending walls which fittingly receive the upper ends of panels38, 40, 50 and 52. As mentioned above, however, it may only be necessaryto coat the exteriorly facing surface of the top.

It will be seen that perishables and ice loaded within the containerwill be adjacent the water-repellant inner surfaces of the container.This restricts penetration of water and other fluids such as blood orjuices into the container. Likewise, the water repellant outer surfaceslimit entry of water and other fluids into the container 12 to avoidcontamination of the contents. The provision of water repellant surfaceson both the interior and exterior of the container thus provides acontainer especially well suited for use where flow of fluid into andout of the container is to be avoided.

It will be appreciated that the use of the corrugated board 10 in thecontainer 12 containing linerboards 12 and 14 coated in accordance withthe invention improves the repulpability of the box so that containersmade from such board may be more readily and more economically recycledthan boxes made of conventionally (wax) coated water-repellantpaperboard, and provides a recycled pulp of improved quality. Thus, ithas been observed that under acidic conditions; i.e., below about pH 7,the ionic cross-linking between adjacent carboxylic acid groups of thepolymer matrix provides a highly water resistant or hydrophobic film onthe board surface which is of sufficient strength and integrity towithstand typical shipping and storage conditions encountered whenshipping ice-packed perishables.

It has also been observed that a board coated according to the presentinvention exhibits improved repulpability as compared withwater-repellant boards such as wax-coated boards and boards coated withpolyethylene resin or extrudable film-forming thermoplastic resins suchas polypropylene, polyamides, and ethylene/vinyl acetate co-polymers aswell as polyolefin coatings, to name a few. It is further believed thatthe network of ionic crosslinking is not as stable to conventionalalkaline media such as sodium hydroxide-based systems used in repulpingas chemically cured compositions; i.e., coating compositions havingcovalent bonds formed by chemical reaction and/or heat. That is, underalkaline conditions; i.e., substantially above about pH 7, andespecially under typical repulping conditions of pH 8 to 11 and 100° F.to 160° F., it is believed that the ionic bonds of the coated board ofthe invention more readily disassociate so that the coating breaks intosmaller fragments faster for enhanced repulpability. The pigments andwax particles in the coating are believed to contribute to repulpabilityby interrupting the continuity or ordering of the cross-linked polymerchains, thereby lowering the cohesive strength of the film so that it ismore readily dispersed during repulping.

In addition to facilitating repulping of the paperboard, the coating ofthe invention is believed to provide improved low slip characteristicsand an increase in the glass transition temperature of the protectivelayer as compared with conventional coated boards of the type used inproviding water-repellant shipping containers, to provide increasedresistance to blocking.

Additionally, board produced in accordance with the present inventionexhibits improved resistance to blocking under warm and humidconditions. This generally improves the handling properties of the boardin relation to stacking of flattened-out boards or blanks and conveyanceof the board through the stages of manufacture, and limits blocking ofadjacent boxes in shipment and storage.

With reference now to FIG. 3, there is shown a diagrammaticrepresentation of the repulping portion of a process for recyclinglinerboard and containers provided in accordance with the presentinvention. In the process, waste paper or board 70 (i.e. dirty stock andalso used boxes and paperboards, as well as papermill broke) in loose orbaled form containing linerboard coated according to the presentinvention is charged, as by conveyor 72 or by hand as is the practice inmany mills, into a conventional pulper known as a hydrapulper shown at74. The hydrapulper 74 contains a sufficient amount of alkali such asNaOH and ammonia to provide a mixture 75 of waste paper and alkalineliquid having a consistency of between about 4 to 18 percent.

The hydrapulper 74 includes an agitator 76, such as a rotor, foragitating the mixture of waste paper and cooking liquor and may alsoinclude a heater such as a steam coil for maintaining a desiredtemperature in the mixture.

In the process, the stock and cooking liquor may be charged to thehydrapulper 74 continuously or on a batch basis and the resultingmixture agitated or subjected to agitation for a time sufficient toaccomplish repulping of the waste paper to the necessary degree. Whenthe stock has been reduced to the desired degree, it passes through aperforated plate 78 in slurry form and the slurry may then be directedfor further processing as by conduit 80, such as to deflakers, secondarypulpers, screens, cleaners and refiners to further defiberize andprepare the pulp for papermaking.

The operating conditions of the pulper 74 are selected in accordancewith the composition of the waste paper, and operation of the pulper forrepulping of the waste paper 70 may be accomplished in a conventionalmanner, but may economically include used coated board as a repulpablematerial with improved results by virtue of the enhanced repulpabilityof the board of the invention. For example, to repulp a board providedin accordance with the present invention which includes wet strengthboard (pulp), the hydrapulper may be operated at conditions common forthe repulping of wet-strength paper, such as the conditions set forthbelow in the first column of Table 1. For non-wet strength board, themilder conditions set forth in the second column of Table 1 may beemployed.

                  TABLE 1                                                         ______________________________________                                        REPULPING WET STRENGTH  NON WET STRENGTH                                      CONDITIONS                                                                              BOARD (PULP)  BOARD (PULP)                                          ______________________________________                                        TEMPERA-  140-160° F.                                                                          80-120° F.                                     TURE                                                                          pH        10-11         7-8                                                   TIME      30-60 min     10-20 min                                             ROTOR TIP 3,500 ft/min  3,500 ft/min                                          SPEED                                                                         ______________________________________                                    

Again, it is believed that the alkaline conditions within the pulpercauses destruction of the ionic cross-linkages between adjacentcarboxylic acid groups of the coating. This reduces the strength of thecoating so that the action of the pulper breaks the coating down intoparticles of reduced size. Also, it has been observed that particlesfrom the coated board of the present invention are "substantially free",which is used herein to mean that the coating substantially separatesfrom the fibers and does not retain a significant amount of adhered orembedded pulp fibers when the board is repulped. This provides animproved recycled pulp/coating particle mixture for use in theproduction of paper products, particularly linerboard for boxboardapplications.

While the mixture of pulp/coating particles is suitable for use in themanufacture of paper products, it will be understood that the mixturemay be added with furnish derived from conventional recycling processes,and new furnish containing virgin pulp. In addition, it will beunderstood that a purer furnish (a higher percentage of pulp) may alsobe accomplished by subjecting the mixture to mechanical separationtechniques (i.e. centrifugal separation) to separate the coatingparticles from the pulp.

The following Examples 1 to 8 illustrate various formulations,embodiments and applications of the invention coatings. In particularExamples 1 to 6 illustrate the aqueous pigment/wax/polymer compositioncoatings on various substrates and their resulting properties andcharacteristics, including repulpability. Example 7 illustrates analternate composition coating without the incorporation of acrosslinking agent and Example 8 illustrates the coating compositions asapplied to preprinted substrates. These examples are merelyrepresentative and are not inclusive of all the possible embodiments ofthe invention.

EXAMPLE 1

A coating for application to a board was prepared by charging 643 lbs ofwater to a Kady Mill, Kinetic Dispersion Corporation, Scarborough, Me.,for high shear mixing. This mixing device disperses the pigmentparticles in water.

3 lbs of tetrasodium pyrophosphate (TSPP) was then added to the mixtureduring agitation. TSPP is a pigment dispersant which keeps the pigmentparticles from settling upon storage and transportation. 135 lbs ofZinplex 15, a zinc ammonium carbonate complex, was then added to themixture as the crosslinking agent for the polymer, i.e. carboxylatedstyrene butadiene latex.

The mixture was then charged with 550 lbs of MicaWhite 200 and stirredfor 10-30 minutes to obtain the proper dispersing of the pigmentparticles.

The high shear agitation blade of the Kady Mill was shut down and themixture was agitated at a low shear using a device such as a lighteningmixer. The mixture was then charged with 1350 lbs of Dow Latex RAP 314NA, a carboxylated styrene-butadiene latex, available from Dow ChemicalCompany. 339 lbs of Nopcote DS101, a synthetic wax emulsion, was addedto the mixture under agitation. Nopcote DS101 is composed of 30-50%synthetic wax, up to 7% emulsifying agents and 50-65% water (% byweight--wet basis). Finally, 6 lbs of Colloid 682 as a defoamer and 35lbs of Admiral 3089FS as a thickener was added to this mixture toproduce the coating mixture.

The resulting coating mixture had a solids content of about 45.2, a pHof about 9.2 to 9.6 and a viscosity between 350-400 cps.

The coating was applied at ambient temperature of about 25° C.-30° C. to57 lb grade HK (Hydrokraft) linerboard (wet strength) in a single passat a coating weight of about 2.5-3.0 lb (dry)/1000 ft² using a MichelmanMRC-1000 rod coater followed by forced hot air and contact drying. Thecoated linerboard was converted to a double-faced corrugated board in aconventional corrugator and the coated board was observed to passthrough the corrugator at a temperature of 350° F. trouble-free. Thestack of corrugated sheets had a temperature of 150°-190° F. duringnormal operation and the corrugated board stacks exhibited nosignificant sticking/blocking after prolonged storage under the stackpressure and temperature.

The finished corrugated sheets were printed with aqueous flexo inks anddie-cut using a rotary printer/die-cutter with a lead edge feeder. Theconverting rate was about 160-200 blanks/minute. Good dry-rub resistancewas observed on the printed surface. The die cut blanks were formed intoboxes.

Fresh chicken breast parts were packed into the boxes and covered withice chips. The boxes were stored in a refrigerated cooler for 4-16 hoursat 38°-43° F. The boxes were then shipped by refrigerated truck for 260miles at 26° F. The boxes performed as well as the conventionallycurtain-coated wax boxes. The board was observed to be readilyrepulpable at 146° F. and pH of 8-11.

EXAMPLE 2

Board produced in accordance with Example 1 (30%) was combined withconventional uncoated board (70%) and defibered in a hydrapulperoperated at 110° F./4-18% consistency/pH 7.6. No operational problemswere observed.

EXAMPLE 3

Coating produced in accordance with Example 1 was applied at atemperature of about 25°-30° C. to either 57 lb PL (pineliner) or 69 K(kraft) non-wet strength linerboard substrates in a single pass at acoating weight of about 2.0-2.5 dry lbs/1000 ft² using a Black-Clawsonair knife coater followed by forced hot air drying. The coatedlinerboard was converted to a double-faced corrugated board in acorrugator and the coated board was observed to pass through thecorrugator at a temperature of about 350° F. trouble-free. The stack ofcorrugated sheets had a temperature of 150°-190° F. during normaloperation and the corrugated board stacks exhibited no significantsticking/blocking after prolonged storage under the stack pressure andtemperature.

The finished corrugated sheets were printed with aqueous flexo inks anddie-cut using a rotary printer/die-cutter with a lead edge feeder.Printed blanks were folded and glued with hot melt adhesives. Theconverting rate was about 200-250 blanks/minute. Good dry-rub resistancewas observed on the printed surface.

Fresh lettuce was harvested from the field, washed, and packaged withwater spray in boxes formed from the corrugated sheets. The boxes werestored in a cooler at 35°-45° F. for a period of 4-12 hours. The boardwas observed to be readily repulpable with uncoated corrugated board(OCC) at 100°-120° F. and pH of 7-8.

EXAMPLE 4

Coating produced in accordance with Example 1 was applied at atemperature of about 25°-30° C. to 57 lb PL (pineliner) and 69 K (kraft)non-wet strength linerboard substrates in a single pass at a coatingweight of about 1.8-2.3 dry lbs/1000 ft² using a Black-Clawson air knifecoater followed by forced hot air drying. In each case the coatedlinerboard was converted to a single-faced corrugated board in acorrugator and the coated board was observed to pass through thecorrugator at a temperature of 350° F. trouble-free. The barrier coatedside was the inside of the box for water/grease resistance and releaseproperties. Each stack of corrugated sheets had a temperature of150°-190° F. during normal operation and the corrugated board stacksexhibited no significant sticking/blocking after prolonged storage underthe stack pressure and temperature.

The uncoated side of the finished corrugated sheets were printed withaqueous flexo inks and die-cut using a rotary printer/die-cutter with alead edge feeder. Printed blanks were folded and glued with hot meltadhesives. The converting rate was about 200-250 blanks/minute.

Fresh meat chunks were cut, washed, and packaged in boxes formed fromthe corrugated sheets. The boxes were stored in a freezer at 15°-30° F.for a period of several hours. The meat chunks were observed to beeasily removed from the boxes and did not stick to the boxes. The boardwas observed to be readily repulpable with other uncoated board at100°-120° F. and pH of 7-8.

EXAMPLE 5

Non-wet strength paperboard (such as 35-69 lbs kraft) produced inaccordance with Examples 3 and 4 was defibered in a laboratory blenderoperated at 100°-120° F./pH of 7-8/4% consistency.

Finished stock was further diluted down to 0.2% consistency with tapwater. Handsheets were made from the stock using a standard TAPPI sheetmold. Handsheets with uniform formation without any stickies andnon-defibered lumps were seen after 40-60 seconds of repulping time.

Conventional curtain-coating wax coated board (non-wet strength) wasdefibered in the laboratory blender under the same conditions.Curtain-coating wax layers were observed to break into small sizeagglomerates, which were visible and not acceptable for papermaking.Non-defibered lumps were also observed in the handsheet.

EXAMPLE 6

Wet-strength paperboard coated in accordance with Example 1 wasdefibered in a laboratory blender operated at 120°-140° F./pH of10-11/2.5% consistency.

Finished stock was further diluted down to 0.2% consistency with tapwater. Handsheets were made from the stock using a standard TAPPI sheetmold. The sheets showed satisfactory defibering and were free from"stickies" after 40-60 seconds repulping time.

Conventional curtain-coating wax coated wet-strength board was defiberedin the same device and conditions for comparison purposes.Curtain-coating wax layers broke into small size agglomerates, whichwere visible and unacceptable for papermaking. Non-defibered lumps werealso observed in the handsheet.

EXAMPLE 7

Coating produced in accordance with Example 1, except that thecrosslinker, Zinplex 15, is excluded. Examples 1 to 6 above wererepeated using this coating without the crosslinking agent. The resultsobtained in each instance were comparable to the coatings including thecrosslinking agent.

As long as the corrugated boards are produced and converted within 2hours, the crosslinking agent i.e. Zinplex 15 is not necessary. If thiscondition is not met blocking occurs which can be problematic.

Generally, blocking, which refers to when corrugated sheets stick toother sheets, is a function of time, temperature, pressure and moisture.Time: If the corrugated sheet stack is converted within two (2) hours,which is a short enough time to prevent any significant blocking undernormal operating conditions, the crosslinker is not needed. Temperature:Generally, during the colder winter season there is less tendency toblock than during the hot, humid summer months. Pressure: If the sheetstack is high, higher pressure is exerted to the bottom of the stack andblocking is more likely to happen than in the situation where there is ashorter sheet stack. Moisture: If the coating is not fully dried, whichmay occur during operation malfunctions, the coating blocks easier.

EXAMPLE 8

Coating produced in accordance with Example 1, except that the coatingis applied to preprinted linerboard substrates.

The preprinted linerboard substrates included clay coated 42 lbs CS(coating smooth or converting smooth); 42 lbs KW (klay white or mottlewhite); 57 lbs PL (pineliner); 57 lbs HK (hydrokraft); 69 lbs IPB(Internatinal Poultry Board); 69 lbs K (kraft); 35 lbs PL (pineliner)and 38 lbs CS (coating smooth) linerboard.

In another series of experiments the preprinted linerboard substrateswere coated instead with the clear, unpigmented, wax/polymer coatingsdescribed in copending Ser. No. 08/128,582.

In each instance the transparent coatings, both pigmented andunpigmented embodiments, in addition to the water vapor resistance,block resistance, water resistance and grease resistance propertiesfurther provides scuff resistance to the coated surfaces.

Scuff resistance of the coatings was determined by visual observationduring the box plant operations. In addition, the Sutherland Rub Testerwas also used to judge the scuff resistance of the overprint varnish. Aheated 4 lb weight (iron) is used in the test at 150°-190° F. to rub onecoated surface against another coated surface. The invention coatingssurvive approximately 75 rubs under such conditions without impairmentto the preprinted surface, which indicates a suitable application as anoverprint varnish.

TABLE II below summarizes some of the physical properties andcharacteristics of the coated substrates of the invention.

                  TABLE II                                                        ______________________________________                                        PHYSICAL PROPERTIES OF COATED SUBSTRATES                                      PROPERTY         MEASUREMENT                                                  ______________________________________                                        COATING WEIGHT   1.2-3.5 lbs/msf                                              COATING THICKNESS                                                                              0.2-1 thousands of an inch                                   RECYCLABILITY AND                                                                              YES                                                          REPULPABILITY                                                                 LOW-SLIP         more than 30 degrees                                         WATER-RESISTANCE 16 hours Cobb <3.5 g/100 in.sup.2                            HOT MELT GLUEABILITY                                                                           YES                                                          FLEXIBILITY      FAIR                                                         BREATHABILITY    --                                                           VAPOR TRANSMISSION                                                                             WVTR of 1-6 g/100 in.sup.2 -24 hrs                                            at 100° F./90% R.H.                                   GREASE RESISTANCE                                                                              6-8                                                          ______________________________________                                    

As discussed earlier in the specification, the coated paper is readilyrepulped and recycled. The coated paper stock can be mixed with regularwaste paper streams such as recycled newsprint and office waste forrecycling purposes. The repulped coated linerboards can be recycledthrough regular cleaning equipment and run through conventional papermachines without any problems.

In the aqueous coatings of the invention the ionic bonds formed by thecrosslinkers are generally dissociated at pH>9-10. After the ionic bondis dissociated the coating is kept separately from the cellulose fibersreclaimed in the repulping process with various cleaning equipments. Thewax and pigment particles formulated in the coatings interrupt thepolymer film structure formed on the paper surface. The film thus breaksdown easier under the rubbing, wearing and shearing actions during therepulping process than the formulations without any pigment and waxparticles at pH 7-8 and 100°-120° F. The coatings provide increasedrecycling capability of the coated materials over the unrecyclable waxboxes.

The low slip surface characteristics of the coated paper surface mayeliminate the plastic stretch wrap typically needed for wax coatedboxes. The slide angle of the wax coated boxes is generally 16-17degrees. The invention polymer/wax/pigment coating formulations providea slide angle of more than 30 degrees, which is enough to prevent theboxes from sliding during shipping and distribution activities.Compositions containing polymer and wax components, as described inco-pending application Ser. No. 08/128,582 filed on Sep. 28, 1993,typically have a high slip with a slide angle of about 9 degrees, whichis advantageous to enhance release characteristics of meat boxes andfacilitate removal of frozen meat from the container.

The water resistance of the coated samples in Examples 1 to 8 weremeasured at 38° F./85% R.H. (relative humidity). The coating weight ofthe samples were in the range of 1.2-3.5 dry lbs/msf.

Using EVA-based (ethylene vinyl acetate) hot melt adhesives such asUPACO 902.9-3 available from UPACO, Richmond, Va., HM60 available fromSouthern Chemical Corp., Mobile, Ala., 3M 3762, available from 3M, St.Paul, Minn., and HM 1828, available from H. B. Fuller, the aqueousbarrier coated paper were found to be hot melt glueable. Both coatedface-to-coated face and coated face-to uncoated face type of glue jointwere done and provided satisfactory results.

The flexibility of the coatings are fair. Under the severe repulpingactions in the hydrapulper the coating is fragile, which is desirablefor recycling. However, the coating is flexible, or tough enough, inapplications for agricultural boxes.

The water vapor barrier level of the coatings are generally between 1-6g/100 in ² --24 hrs at 100° F./90% R.H. Thus the coatings are notconsidered "breathable" but rather are a moisture vapor barrier film.

The kit, grease resistance, values are generally dependable on thecoating thickness applied to the substrate surface. Typically the kitvalues for the composition coatings range from 6-8, which provideacceptable levels of grease resistance

The coating compositions of the invention as discussed earlier may beapplied by a number of different methods. In addition they may beapplied either on-corrugator or off-line. In on-line corrugatorapplication the coatings can be applied on both the wet- and dry- end ofthe corrugator. This provides cost savings compared to an off-linecoating application. These processes however are more difficult tocontrol than the off-line applications.

In the wet end application process, the coating can be applied and driedprior to the single facer and after the double backer. Additional dryingequipment is required for this type of application method. In the dryend application process, coating can be applied after the hot platesection. Residual heat in the constructed corrugated board dries thecoating. Equipment used for wet end applications can include a rodcoater, i.e. MRC 1000, and a blade coater, WEC-130S and WEC-127, fromMichelman, Inc. Blade coaters such as DEC-500 and PDD/4 are designed fordry end applications by Michelman, Inc.

Various off-line coaters such as a rod coater, blade coater and airknife coater can be used to apply the invention coatings. The coating ispreferably dried thoroughly after the application in order to preventany sticking or blocking within the coated rolls. Coated linerboardrolls are later run on a corrugated line to form the constructed board.

The present invention provides advantages over prior practice inproviding a paperboard which can be recycled by conventional methodswithout any special measures or equipment. The aqueous coatings of theinvention break up and disperse with the paper fibers when the materialis run through a standard hydropulper under alkaline conditions.

It will be recognized by those skilled in the art that the compositionsand coated stock of the invention and process have wide application inthe production of packages or containers having water resistance, lowslip, hot melt glueability, grease resistance and moisture vapor barriercharacteristics. Advantageously, the coating compositions may also beused as overprint varnishes on preprinted linerboard substrates toimpart additional scuff resistance to the coated substrate surface.

The foregoing description of various and preferred embodiments of thepresent invention has been provided for purposes of illustration only,and it is understood that numerous modifications, variations andalterations may be made without departing from the scope and spirit ofthe invention as set forth in the following claims.

We claim:
 1. A water-repellant paperboard comprising a fibrous webhaving a pin-hole free continuous film coating which is water resistantand repulpable;said coating consisting essentially of pigments, a waxcomponent selected from the group consisting of paraffin wax andpolyethylene wax emulsions and mixtures thereof, and a polymer matrix ofpolymer chains comprising a polystyrene-butadiene polymer copolymerizedwith a monomer having carboxylic acid pendant groups wherein saidpendant carboxylic acid groups of said polymer chain are crosslinked bya zinc ions ionically bonded between carboxylic acid groups of adjacentpolymer chains; wherein said pigments are selected from the groupconsisting of aluminum trihydrate, barium sulfate, calcium carbonate,mica (potassium aluminum silicates), nepheline syenite (sodium potassiumaluminosilicate), finely ground silica sand and other natural andsynthetic type of silicates, talc (magnesium silicates), wollastonite(calcium metasilicates), bentonite (montmorillonite, smectite) and clay;and said coating has a dry coating weight in the range of 1.2 to 3.5lbs/msf.
 2. The paperboard as defined in claim 1, wherein said fibrousweb is made into wet strength or non-wet strength board.
 3. Thepaperboard as defined in claim 1, wherein said crosslinking agent isselected from the group consisting of zinc oxide ammonium complexes. 4.The paperboard as defined in claim 1, wherein said pigment and waxparticles decrease the cohesive strength of said polymer matrix.
 5. Thepaperboard as defined in claim 1, wherein said coating is applied as anaqueous formulation to said fibrous web and said wax is provided in theformulation as an emulsion.
 6. A coating formulation for application toa surface of a cellulosic substrate to provide a pin-hole freecontinuous film water-repellant coating which consists essentially of:apolystyrene-butadiene polymer copolymerized with a monomer havingcarboxylic acid pendant groups, pigments, a wax component selected fromthe group consisting of paraffin wax and polyethylene wax emulsions andmixtures thereof, and a cross-linking agent which provides zinc ions;wherein said crosslinking agent reacts with said polymer to provideionic cross-linking between the carboxylic acid groups of thecarboxylated polymer to create an ionically cross-linked polymer matrixin the resulting coating; wherein said pigments are selected from thegroup consisting of aluminum trihydrate, barium sulfate, calciumcarbonate, mica (potassium aluminum silicates), nepheline syenite(sodium potassium aluminosilicate), finely ground silica sand and othernatural and synthetic type of silicates, talc (magnesium silicates),wollastonite (calcium metasilicates), bentonite (montmorillonite,smectite) and clay.
 7. The coating formulation as defined in claim 6wherein the cross-linking agent is selected from the group consisting ofzinc oxide ammonium complexes.
 8. The coating formulation as defined inclaim 6, wherein the formulation comprises:25-60% of said pigments;25-45% of said polymer; 10-30% of said wax; and 2-10% of saidcross-linking agent.
 9. The coating formulation as defined in claim 8,wherein the formulation further comprises defoamers, thickeners and/orpigment dispersants.
 10. The coating formulation as defined in claim 6,wherein said substrate is paperboard.
 11. The coating formulation asdefined in claim 6, wherein said substrate surface is preprinted. 12.The coating formulation as defined in claim 6, wherein said formulationhas a viscosity in the range of 150-2500 cps and a solids contentgreater than 35%.
 13. A method of making a water-repellant fibrous webwhich comprises:providing a fibrous web; applying to the surface of saidweb an aqueous coating which comprises a polystyrene-butabiene polymercopolymerized with a monomer having carboxylic acid pendant groups,pigments, a wax component selected from the group consisting of paraffinwax and polyethylene wax emulsions and mixtures thereof, and across-linking agent which provides zinc ions; and reacting thecross-linking agent with the carboxylic acid groups of the polymer tocause the formation of ionic cross-linkages between the carboxylic acidgroups of adjacent polymer chains and provide a substantially pin-holefree continuous coating on the surface of the web; wherein said pigmentsare selected from the group consisting of aluminum trihydrate, bariumsulfate, calcium carbonate, mica (potassium aluminum silicates),nepheline syenite (sodium potassium aluminosilicate), finely groundsilica sand and other natural and synthetic type of silicates, talc(magnesium silicates), wollastonite (calcium metasilicates), bentonite(montmorillonite, smectite) and clay.
 14. The method according to claim13, wherein said aqueous coating comprises:25-60% of said pigments;25-45% of said polymer; 10-30% of said wax; and 2-10% of saidcross-linking agent.
 15. The method according to claim 13, wherein saidcrosslinking agent is selected from the group consisting of zinc oxideammonium complexes.
 16. The method according to claim 13, wherein saidfibrous web is made into wet strength board.
 17. The method according toclaim 13, wherein said fibrous web is made into non-wet strength board.18. The method according to claim 13, wherein said surface of saidfibrous web is preprinted.
 19. The method according to claim 13, whereinsaid aqueous coating has a viscosity in the range of 150-2500 cps and asolids content greater than 35%.
 20. The method according to claim 13,wherein said coating has a dry coating weight in the range of 1.2 to 3.5lbs/msf.